What is BMI
The Body Mass Index (BMI) is a numerical value derived from the ratio between body weight and the square of height. Introduced by Belgian mathematician Adolphe Quetelet in the 19th century, it is adopted by the WHO as a screening tool to classify weight in adult populations.
Despite its simplicity and wide use, BMI does not distinguish between fat mass and lean mass: a very muscular person could be classified as overweight while being in perfect health. It should therefore be interpreted as a population-level risk indicator, not as an individual diagnosis.
The BMI formula
The calculation is straightforward: BMI = weight (kg) ÷ [height (m)]². For example, a person weighing 70 kg and 1.75 m tall gets a BMI of 70 ÷ (1.75 × 1.75) = 22.9. In imperial units the formula is BMI = (pounds ÷ inches²) × 703.
How to interpret the values
The WHO defines these ranges for adults (18+ years):
- Underweight: BMI < 18.5
- Normal weight: BMI 18.5 – 24.9
- Overweight: BMI 25 – 29.9
- Obesity class I: BMI 30 – 34.9
- Obesity class II: BMI 35 – 39.9
- Obesity class III (severe): BMI ≥ 40
Limitations of BMI
BMI does not account for the distribution of body fat, which is a more significant cardiovascular risk factor than absolute weight. Visceral (abdominal) fat is associated with greater metabolic risks than subcutaneous fat, but BMI does not distinguish between them.
Categories for which standard BMI is less reliable: athletes with high muscle mass, older adults, pregnant women, and children (who require age- and sex-specific percentiles). For these people, bioelectrical impedance analysis or waist-to-hip ratio measurement is preferable.
Why BMI remains useful
Despite its limitations, BMI is an inexpensive, quick, and non-invasive tool. Studies show a statistically significant correlation between high BMI and the risk of type 2 diabetes, hypertension, cardiovascular disease, and some cancers.
Use it as a starting point for a conversation with your doctor, not as a definitive judgment on your health. Combine it with waist circumference and other clinical parameters for a more complete picture.